Battery charging technologies are an important part of the development of new electric battery-powered devices, such as electric vehicles. For example, environment-friendly and energy-saving new energy electric vehicles are an emerging field of automobile development. Batteries within electric vehicles or other electric devices may be composed of a plurality of battery modules connected in series. For example, if a battery includes 10 battery modules in total, each battery module providing 40V voltage output, then a 400V voltage output may be obtained when all the battery modules are connected in series. In some charging systems, such battery modules may be connected in series to charge all of the modules together. However, in some such cases, battery modules charged in series may charge unbalancedly and certain individual battery modules might not fully charge, for example, due to differences in resistance and the like among the individual battery modules.